Electrolytic plating method

ABSTRACT

A METHOD OF IMPROVING THE ELECTROPLATING OF A METAL ONTO THE SURFACE OF A CATHODIC STRIP OF METAL WHICH IS MOVED IN THE ELECTROLYTE IN PROXIMITY TO THE SURFACE OF AN ANODE, SAID METHOD INVOLVING THE CREATION OF SUBSTANTIALLY UNIFORM UPWARD FLOW OF ELECTROLYTE ACROSS THE SURFACE OF SAID STRIP OF METAL USING GAS EVOLVED BETWEEN THE ANODE AND SAID STRIP OF METAL AS THE DRIVING FORCE FOR A PUMPING SYSTEM TO CREATE AN AIR LIFT EFFECT. APPARATUS FOR CARRYING OUT SAID METHOD.

March 2, 1971 C, B, YATES I 3,567,595

ELECTROLYTIG PLATIN@ METHOD dws )lles A OPA/[XS March 2, 1971 Q B, YATES ELECTROLYTIC PLATING METHOD 3 Sheets-Shet 2 med seht. 25, 1967 INVENTOR.

550W/e5 )les March 2, 1971 C. B, YATES 3,561,595

ELECTROLYT IC PLATING METHOD Filed Sept. 25, 1967 3 Sheets-Sheet l Arr Afa/s Unted States Patent O 3,567,595 ELECTROLYTIC PLATING METHOD Charles B. Yates, Willingboro, NJ., assignor to Circuit Foil Corporation, Bordentown, NJ. Filed Sept. 25, 1967, Ser. No. 670,152 Int. Cl. B01k 3/00; C23b 5/58 us. cl. 2in- 2s 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an electrolytic process and the apparatus used therein and more particularly to a process and apparatus for continuously electroplating a moving strip of metal.

Electroplating techniques are known in which a continuously moving metallic sheet is moved in serpentine fashion into and out of an electrolyte between successive planar anodes vertically arranged in parallelism with one another. By rendering the metallic sheet cathodic, metal is electrodeposited in the sheet as it traverses the electrolyte. A system of this type is illustrated and described in copending application Ser. No. 421,048, filed Dec. 24, 1964 and now abandoned.

In such an electroplating process, it is highly desirable that the metallic deposit be of a very high degree of uniformity of thickness. Two problems are encountered in conventional methods which result in variations in coating thicknesses which are inadequate. First, electrical current which flows from the cathode to the anode in an electroplating process often leaves the desired plating surface in a non-uniform manner, which results in uneven current distribution and plating. In addition, the weight of the deposited material often varies due to changes in the deposition current efficiency resulting from an ionic imbalance in the electrolyte bath. This ionic imbalance has been found to occur because of variations in degree in flow and agitation in the electrolytic bath during the deposition process.

Problems such as the foregoing become particularly troublesome in certain electrolytic processes such as, for example, one involving the electrodeposition of chromium on a moving metallic sheet. For some electrolytic systems, establishing a uniform current distribution is adequate to obtain an adequate electrodeposit. In chrome plating, however, this is not enough. More specifically, the common chrome plating baths in commercial use have current efficiencies between 20 and 35% and the efficiency has been shown to be related to a wide collection of bath variables such as chromate to sulfate ratio, foreign metal contamination, temperature, agitation, degree of physical electrolye exchange from cathode to anode, and so forth. In order to achieve a uniform chrome deposit, it is necessary for the electrolyte bath to behave uniformly with respect to current efficiencies of deposition at different parts of the cathode. Thus, the uniformity of the flow and agitation pattern in the electrolyte is of very great importance in achieving a uniform thickness of chrome deposit.

The importance of a uniform chrome layer takes on added significance when considering the use of a product such as chrome plated copper foil as a bimetallic lithorice graphic or offset plate. In processing such a plate, starting with a chrome plated copper sheet, part of the chrome surface is masked with a photosensitive polymer leaving the areas of chrome which it is desired to remove exposed, such that they may be attacked by an etchant. As the chrome is attacked in the etching step, there is a tendency for the etchant to attack sideways as well as vertically causing the area of the base copper which is finally exposed to be somewhat larger than the area of chrome originally exposed. This problem is called dot growth, its name coming from the manifestation of the effect in the etching of half tone dots where the eye has been shown to be sensitive to infinitesimal variations in dot diameter which in turn cause variations in the shading or color density of an area.

In developing transparent positives for use in bimetal plate making, the camera attendant normally adjusts the dot size of the transparency to allow for a given degree of dot growth. The problem arises in that the degree of dot growth is a direct function of the chrome thickness, and chrome plates made in the conventional sheet by sheet manner suffer from all of the undesirable edge effects and non-uniform flow and agitation effects of a still bath. Thus, the printer in using such plates is forced to accept an undesirable degree of variation in dot growth which, in multicolor work, seriously diminishes the quality of the result. In this particular usage, uniformity of chrome thickness assumes a unique degree of importance not often necessary in other sorts of chrome plating.

The present invention provides a process and apparatus for continuously electroplating a sheet of metal foil wherein the above-mentioned disadvantages are eliminated. An even deposition of the electrodeposited layer is obtained by utilizing a novel electrically insulating shield and anode barrier arrangement in the plating tank which permits the control of the flow and agitation of the electrolytic bath during deposition.

It is accordingly a primary object of the present invention to provide a novel continuous electroplating process which is extremely effective to form high quality, uniformly thick, metallic electrodeposits on a sheet of base metal.

It is a further object of the present invention to provide a novel electroplating process and apparatus for carrying out such process whereby a sheet of metal may be continuously electroplated in which the electrolyte circulation is controlled by a unique anode and insulating barrier construction.

It is still another important object of the present invention to provide a novel anode structure which makes possible the obtaining of electrodeposits which are extremely uniform in thickness.

These and further objects and advantages of the present invention will become more apparent through reference to the following description and appended claims, as well as to the accompanying drawings wherein:

FIG. l illustrates a longitudinal section of an electrolytic apparatus in accordance with the present invention taken along line 1 1 in FIG. 2;

FIG. 2 is a plan view partially in section of a portion of the apparatus in FIG. l;

FIG. 3 is a fragmentary sectional perspective view of a portion of the electrolytic apparatus of FIG. l;

FIG. 4 is a perspective View of the anode used in the present invention; and

FIG. 5 illustrates in schematic form part of a plating bath in the area near the edge of an anode and a cathode.

As previously indicated, the novel method and apparatus of the present invention are applicable to the electroplating of any metal on a base metal where uniformity of thickness of the electrodeposit is desired, such as copper on copper, chrome on copper, copper on steel, or the like. Since such novel method and apparatus are particularly advantageous in chrome electroplating systems, however, the ensuing description will be couched in terms of the electroplating of chrome on a continuous strip of copper, though the broad essence of the invention is not to be deemed as being so restricted.

The electroplating apparatus illustrated in FIG. 1 comprises a tank containing an electrolyte inlet y12 and outlet 14 for entrance and withdrawal of electrolyte 16 from the tank 10. A positive bus bar 18 is secured to one longitudinal edge of the tank 10 by means of an insulating strip 20. Supported on the bus bar 18 and insulating strip 20 by means of copper bars 22 are a plurality of anodes 24. These anodes 24 are shown in the form of thick planar lead plates, whose physical configuration forms an important feature of the present invention, as hereinbelow described. A terminal lead 2S, adapted to be connected to the positive terminal of a power source (not shown), is connected to the bus bar 118. A support frame 26 is mounted on the tank 10. In order to guide a sheet of copper 54 to be plated, a series of upper electrically conductive idler rollers identified by the numbers 28, 30, 32, 34 and 36 are mounted on and are in electrical contact with the support frame 26, and a series of polyvinyl chloride idler rolls 38, 40, 42 and 44 are rotatably mounted on stainless steel bars 46, 48, and 52, which in turn are connected to the supporting frame 26 at their upper ends. By virtue of the electrical contact between rollers 28, 30, 32, 34 and 36 and support frame 26, and by connecting the latter to the negative terminal of a power source (not shown), copper sheet 54 may be rendered cathodic.

The continuous sheet of copper S4 which is to be electroplated is shown fed from a supply spool 56 mounted on the support frame 26, over the first upper idler 36 and into the electrolytic bath 16. The continuous sheet 54 is guided into and out of the electrolyte 16 in serpentine fashion by means of the remaining idler rollers 38, 40, 42 and 44 at the bottom of the tank 10 and the upper idler rollers 28, 30, `32 and 34. As the plated sheet 54 emerges from the electrolytic tank 10 it passes by a pair of nozzle elements 62 and 64 which are connected to a source of wash solution (not shown) to permit the sheet 54 to be spray washed on both sides. The plated sheet metal then is fed across idler roller 66, past a squeegee device 68 which removes any excess material, past a pair of infrared lamps 70 and 72 to dry the plated material, onto an additional idler roller 74 and finally to the take-up roll 76 which is provided with a suitable drive mechanism (not shown) which provides the necessary force to draw the sheet of copper 54 through the system.

The system thus far broadly described is conventional. Details of the essence of the present invention are shown specifically in FIGS. 2-4. FIG. 2 illustrates a section of the electroplating system as viewed from the top. This section includes an anode 24, a cathode (which is the copper sheet 54 to be plated), a bottom idler roll 38 and a bottom roll support tube 46.

Secured to each side edge of anode 24 at 92 is one leg of a reverse S-shaped barrier 90, which is preferably constructed of a non-conductive material such as polyvinyl chloride. The other leg of each barrier is secured at 94 to one edge of a back shield 96 which is preferably constructed of polyvinyl chloride or similar non-conductive material. Back shield 96 is mounted by means of nonconductive brackets 98 (preferably made of polyvinyl chloride) each of which has a slot 100 which engages an edge of back shield 96. Each of brackets 98 is, in turn, secured to a support tube 46 at 99. As shown in FIG. 2, the edges 101 of copper sheet 54 are positioned within a channel 103 formed within barrier 90 but are not secured to barrier 90 so that movement of sheet `54 will not be obstructed.

As will be readily seen from the drawings, by virtue of the presence of barriers 90, the electrolyte between each anode 24 and the portion of sheet 54 which is parallel to and adjacent the planar face of the anode is effectively segregated from the remainder of the electrolyte in tank 10 to form channels 102 which are confined by four walls (barriers 90, an anode 24 and sheet 54) but which are open in a slot of uniform width at the bottom and open at the top. By virtue of the parallelism between anodes 24 and the adjacent portions of sheet S4, confined channels 102 each have a substantially constant crosssection from the bottom to the top of anodes 24. As will be seen hereafter, the establishment of these confined channels 102 forms an essential aspect of the present invention.

Details of anodes 24 are shown in FIG. 4. Each anode 24 is comprised of a slab of conductive material (preferably lead in a system in which chrome is plated on a copper sheet) and is provided with a pair of conductive projections 22 (which may appropriately comprise the ends of a single copper bar) which are used to mount anode 24 on bus bar `18 so as to establish an electrical connection between the power source and the anodes. Positioned near the top of each anode 24 but slightly below projections 22 is a longitudinal and horizontally eX- tending slot 110. Reinforcing bars 112 mounted over bracket slot provide structural strength to the anode. 'Secured to one face of each anode 24 by conventional means (not shown) is a non-conductive sheet 113, which may appropriately be made of polyethylene.

The operation of the system of the present invention may be described as follows. A sheet of metal 54 of suitable width is fed from supply roll 56 into and out of the electrolyte solution 16 over the upper idler rollers 28, 30, 32, 34 and 36 and the lower rollers 38, 40, 42 and 44, past idler rollers 66 and 74 to the take-up reel 76. Upon energizing the system, the motor driven take-up reel 76 drives the sheet of metal 54 at a desired speed through the electrolytic bath 16 while a positive voltage is applied to the anodes 24 and a negative voltage to the metal sheet 54 acting as a cathode.

In order to achieve uniform deposition of the chrome layer on the copper sheet 54, it is essential that a uniform flow distribution of electrolyte material between the cathode and anode be achieved. Because of gas generation which takes place at the anode interface (generally shown by the bubbles at in FIG. 3), the channel 102 between the anode 24 and the copper sheet 54 is lled with a foamy solution whose average density is less than the average density of the remainder of electrolyte 16. This causes a Pohle air lift effect which carries the liquid electrolyte ,16 up towards the top of anodes 24. By mounting anodes 24 so that the lower edge 114 of slot 110 is substantially level with the surface of the electrolyte 16, the foamy electrolyte is caused to rise up in channel 102 and to flow through slot 110 as shown in FIG. 3. The result is that a circulation pattern is formed from the bottom to the top of channel 102 and through slot 110 of anode 24.

`Because this circulation pattern is confined within channel 102, the pumping action of the generated gas causes the whole electrolyte body 16 to move upwardly at substantially the same velocity across the width of the metal sheet 54, and to flow out over the anode 24 through the slot `110 at a uniform rate across the entire width of the anode 24. This creates a uniform flow pattern in the desired direction, with no eddies and no still zones. All of the agitation and pumping effects of the generated gas occur in the same manner and cause the same results across the entire width of the plating area of the metal sheet 54 because the flow pattern is entirely unidirectional and there is no factor or effect which would tend to disturb this uniformity. The result is uniform thickness of deposit across the width of sheet 54.

By virtue of the unique arrangement described above,

the nature of the electrodeposit on the metal sheet can be readily modified by control of electrolyte circulation within confined channel 102. Since the electrolyte circulation within channel 102 is directly related to the amount of electrolyte which is caused to flow through slots 110 of anodes 24 as a result of the gassing of the electrolyte as aforesaid, the electrolyte circulation can be modified by adjustment of the position of slots 110 relative t0 the electrolyte surface. By raising anodes 24 above the level of the electrolyte solution 16, the flow of electrolyte over the anode slot 110` may be impeded somewhat. By bringing the anode 24 down below the level of the electrolyte solution 16, it is easier for the electrolyte to be lifted by` the gassing effect and to pass through the slot 110, thereby increasing electrolyte ow. Thus, the ow rate in the plating area can be readily adjusted by adjusting the height of the anode 24.

The exact position of the anode 24 must be determined for each specilic application and depends on a number of variables including the distance between the cathode and anode, the materials utilized, the electrolyte solution, the rate of movement of the sheet of material to be plated and the thickness of the plating desired. If the anode is placed too high above the level of the electrolyte solution 16, there will not be a suicient flow rate and the electrolyte will become depleted in terms of its chromatc content (in systems in which chrome plating is taking place), which will result in a loss of current efficiency and produce poor electroplating. If the anode 24 is placed too far below the level of the electrolyte solution 16 there will be virtually a still ow and the gasses will come up and simply bubble out of the electrolyte and not cause the desired lifting effect of the electrolyte across slots 110 of the anodes 24.

In the preferred embodiment, the anode 24 has been shown with a slot 110 through which the electrolyte solution 16 flows, with separate projections 22 being provided above the lower edge 114 of slot 110 for mounting of the anodes. This particular construction facilitates the handling and mounting of the anode 24 and eliminates the necessity for sealing of the tank at the points at which projections 22 come in contact with the supporting tank portion. While this arrangement is thus highly advantageous, the broad advantages of the present invention may still be achieved through the use of an anode without a slot by mounting such anode with its upper edge substantially level with the electrolyte surface. In such case, the upper edge of the anode performs the function of the lower edge 11-4 of slot 110. As will be apparent, this arrangement requires that special precautions be taken to seal the tank at the points at which the anodes are mounted.

As will be apparent from the preceding description, the arrangement of the present invention is not only effective to establish uniform electrolyte circulation patterns between the anodes and adjacent moving cathodic metallic sheets (resulting in extremely uniform electrodeposits) but provides a significant secondary advantage. This secondary advantage results from the use of barriers 90 and their associated back shields 96 and can best be explained by reference to IFIG. 5, which illustrates schematically current ow between a cathode and anode in an electrolyte both with no edge shield (such as barrier 90) being present. In FIG. 5, the path of the current across the bath is represented by dotted lines 200 whose spacing indicates the current density across the electrolyte. In the right-hand area 202 of the sketch, where vast continuous areas of anode and cathode are exposed to one another at a uniform spacing, a uniform current density exists in the electrolyte assuming that the resistivity of the electrolyte is the same throughout its volume. In the area of the edge 204 of the sheet, the current will proceed directly across the electrolyte in the same manner and with the same density as in the right-hand area, but in addition to that current, the volume of the electrolyte to the left of the edge not directly between the anode and cathode is also capable of conducting current, and additional current will leave the anode in the area near the edge to take advantage of the conductivity of this extra volume of electrolyte. This extra flow of current is represented in FIG. 5 as dotted lines 206 extending well out into the side body of electrolyte. The conducting path may be somewhat longer, and as a result the actual current density across the electrolyte will fall olf dramatically as one moves away from the anode edge to the left into the electrolyte volume, but the net effect as far as the cathode is concerned is a considerably higher total amount of current discharged at that edge, and consequently a thicker deposit.

In order not to confuse separate effects, the current lines which would normally emanate from the back of the j anode which is also at a potential relative to the cathode and the electrolyte, and which would also inevitably be the source of additional current) have not been shown in FIG. 5. This additional current, due to the greater exposed anode area which is often assumed to be the sole cause of edge build-up, is really only one of several contributing factors. For example, if the anode were insulated on its back and along its edges, entirely masking all of its surface other than that facing the cathode, there would still be a considerable edge build-up due to the greater volume of electrolyte available for carrying current near the edges of the electrodes.

In the system of the present invention, barriers y cut off all extraneous current paths immediately before the edges of the moving cathodic sheet 54, so that current flow is not permitted to loop out into the side body of electrolyte. Except for a very narrow band of no more than 1-2 cm. in width, therefore, the edges of the sheet will see no more current flow than the center of the sheet. In addition, the back shields 96 will serve as a rear current barrier to prevent electrodeposition on the rear surface of sheet S4. These functions are, as indicated, performed in addition to the primary function of establishing uniform electrolyte circulation patterns in conjunction with the unique anode of the present invention.

As previously indicated, each of anodes 24 is provided with a non-conductive sheet 113 on one of its faces. This non-conductive sheet has the effect of making each anode a one-sided anode so as to avoid gas evolution on both sides of the anodes. Unless this were done, the air lift effect on one side of a given anode would cancel out that on the reverse side and the advantages of the present invention would not be obtained. A similar effect could be obtained from placing two non-insulated anodes back-toback in order to create a still volume of electrolyte between them.

The operating conditions employed in the carrying out of the process of the present invention will vary through wide limits depending on a number of variables including but not restricted to the nature of the base metal as well as plating metal, the thickness of the base metal, the thckness, smothness and uniformity of the plating metal which is desired, the speed of movement of the base metal sheet throughout the electrolyte, and so forth. Those skilled in the art will have no diiculty adapting the proper conditions of the present invention to the production of a particular bimetallic product.

By way of example of the practice of the present invention, a matte chrome surface may be electrodeposited on a moving copper sheet approximately 60 inches wide using an electrolyte approximately as follows:

Bath composition: Grams per liter Chromic acid 200 Sulfuric acid 2 Utilizing the above electrolyte, the following approximate operating conditions may be employed:

Temperature-20-29 C. (preferably 25 C.) Cathode current density-400 amps per square foot r Average tank voltage--6-8 volts Anode-cathode spacing-1%: inch Sheet speed-8 inches per minute Bath time-2-4 minutes Under the operating conditions set forth above, the thickness of a chrome plating on a copper sheet mils thick will be approximately 2 microns.

The circulation of the electrolyte within the tank (other than that provided by the novel barrier-anode arrangement previously described) need not exceed more than 2 or 3 gallons per minute. This fiow is sufiicient to keep the electrolyte from being depleted but does not affect the flow characteristics caused by the Pohle air lift effect described above.

By means of the novel process and apparatus of the present invention, metal sheets can be produced having a continuously electrodeposited coating the thickness of which is extremely uniform across the entire width of the sheet. For example, while prior art techniques often result in coating thickness variations across the width of the sheet which vary 100% or more, the present invention can reduce this thickness variation figure to 50% and even to 30% and less.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment and processes are therefore to be considered in all aspects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed is:

1. In a process for electroplating a metal from an electrolyte solution onto the surface of an elongated strip of metal in which said strip of metal is in contact with an electrically conductive surface rendering the strip of metal cathodic in the electrolytic circuit, said strip of metal being moved through the electrolyte in proximity to at least one anode immersed in said electrolyte, said anode causing the evolution of gas in the electrolyte between said moving strip of metal and the side of the anode fac-ing said strip of metal, the improvement comprising: creating substantially uniform upward fiow of electrolyte past the surface of and substantially throughout the width of said strip of metal using said evolved 4gas as the driving force for a pumping system to create an air lift effect.

2. A process as defined in claim 1 wherein said air lift effect is of the Pohle type.

3. A process as defined in claim 1 wherein said substantially uniform upward iiow of electrolyte is created by confining the electrolyte between said strip of metal and said anode to prevent the migration of electrolyte transversely across the edges of said strip; and maintaining the relative positions of the electrolyte surface and the anode so that electrolyte is caused by the gas evolution to circulate up over a portion of the anode adjacent said electrolyte surface so as to obtain a substantially uniform circulation pattern within said confined body of electrolyte.

4. A process as defined in claim 3 wherein said electrolyte is confined by positioning a non-conductive barrier at each edge of said strip of metal and extending between said strip of metal and said anode.

5. A process as defined in claim 3 wherein said anode is mounted so that an exposed upper peripheral surface is positioned substantially level with the surface of the electrolyte.

6. A process as defined in claim 3 wherein a plurality of substantially vertically arranged, substantially planar anodes are immersed in said electrolyte and `wherein said strip of metal is moved through said electrolyte in serpentine fashion so as to travel alternatively upwards and downwards between successively arranged anodes. 2 7. A process as defined in claim 3 wherein said confined body of electrolyte between said strip of metal and said anode has a substantially constant cross-section in a direction parallel to the direction of movement of said strip of metal.

8. A process as defined in claim 3 wherein said metal to be electroplated is chromium.

9. A process as defined in claim 3 wherein said anode is mounted so that an exposed upper peripheral surface is positioned substantially level with the surface of the electrolyte and wherein a plurality of substantially vertically arranged, substantially planar anodes are immersed in said electrolyte; wherein said strip of metal is moved through said electrolyte in serpentine fashion so as to travel alternatively upwards and downwards between successively arranged anodes; and wherein said confined body of electrolyte between said strip of metal and said anode has a substantially constant cross-section in a direction parallel to the direction of movement of said strip of metal. References Cited UNITED STATES PATENTS 2,317,242 4/1943 Allen, et al 204-206 3,104,221 9/1963 Hill 204-237 1,794,973 3/1931 McBride 204-28 2,461,556 2/1949 Lorig 204-28 3,081,238 3/1963 Gurry 204-28 3,316,160 4/1967 Uchida et al 204-28 JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner U.S. Cl. X.R. 

